An energy harvester is a device that converts mechanical movements into electrical energy. This electrical energy can then be stored or used by other devices. Thus, an energy harvester of this type can utilize energy that would otherwise be lost. For example, the vibrations of an air duct can be converted to electrical energy by an energy harvester and the electrical energy can be then be used to power a sensor that measures the temperature of air in that duct. Therefore, the sensor will not require periodic battery changes.
Applications of such energy harvesters include supplementing other power sources or recharging batteries, thereby extending battery life; elimination of wiring for electrical devices remote from a power source, the powering of mobile electronic instruments and powering wireless monitoring applications. These latter applications typically comprise the sensing of local conditions to generate monitoring data, optionally, the processing of that data and the wireless communication of the data to a central data processing point. Such applications include wireless health monitoring of machine, human or vehicle systems, wireless monitoring of temperature, air or fluid flow, humidity, and gas content in heating, ventilation and air-conditioning (HVAC) systems; wireless monitoring of traffic flow, turbulence, noise, troop or other personnel movements; wireless, self powered security systems; and systems including passive detection of creep or crack propagation in structures to allow for “condition-based maintenance.”
There are a variety of conventional devices for generating electrical power from vibrations, oscillations or other mechanical motions. These devices include inductive devices, capacitive devices, and piezoelectric devices. Capacitive devices make use of the capacitor equation:
      Q    ⁡          (      t      )        =      CV    =                  κɛ        o            ⁢                        A          ⁡                      (            t            )                                    d          ⁡                      (            t            )                              ⁢      V      The devices are arranged so that external vibrations vary the capacitor plate overlap area (A) and/or the capacitor plate spacing (d). Thus, a vibration causes a change in charge on the capacitor when a voltage is applied to the device. When the capacitor is used to drive a load, the charge flow is damped with a characteristic decay time given by the time constant, τ=RC. These capacitive devices generate an electrical signal that varies with a frequency that is the same as the vibration frequency, but require an input voltage.
Piezoelectric materials generate a voltage when they are stressed in accordance with a piezoelectric equation:Vi=gijpiezoσj·di where gij is a coefficient that describes the ability of the piezoelectric material to convert a stress in coordinate direction j, to a voltage in direction i, σj is a Cartesian component of stress applied to the piezoelectric material, and di is the spacing between electrodes that measure the voltage, Vi in the same direction. Ceramic (polycrystalline) piezoelectric elements, flexible piezoelectric fiber composites, or polymeric electroactive materials can be used in various energy harvesting applications. One proposed class of electroactive energy harvesters makes use of the periodic compression in the heel of a shoe or boot caused by walking to stress a piezoelectric material in order to generate power. Walking generates a stress on the order of 200 lbs. over 10 in2 or 1 to 2×105 Pa. With piezoelectric stress coupling coefficients typically in the range 5-20 millivolt/(meter-Pa), the voltage generated by a piezoelectric energy harvester in such an application would be of order 1.3 volts, with a power density (½CV2)ω on the order of 2/R watts/cm3. However, because this device is limited in its charge, the load resistance must be high to prevent rapid loss of charge. Hence the power density is small, typically measuring in the μW/cm3 range.
In a real device constructed with piezoelectric polymers, walking impact generated an average power of approximately 8 milliwatts (“Energy Scavenging with Shoe-Mounted Piezoelectrics”, N. S. Shenck and J. A. Paradiso, IEEE Microelectronics, v. 21, n. 3, May-June 2001, p. 30-42) corresponding to a power density of order 1 mW/cm3. Another device using piezoelectric fiber composites projects an ultimate average power density of approximately 0.1 milliwatts/cm3 (“Compact Piezoelectric Based Power generation”, K. Ghandi, Continuum Controls, Inc., DARPA Energy Harvesting Program Review, 2000). The small size of these devices puts their energy densities in the range of 0.1 to 1.0 milliwatts/cm3 with projections up to 5 milliwatts/cm3.
Inductive devices that convert vibrations to electrical power essentially work like an acoustic speaker (in which electrical signals are converted into vibrations of the speaker cone) in reverse. This operation can also be considered on the basis of the generator principle, that is, Faraday's law of induction:
      V    ⁡          (      t      )        =      N    ⁢                  ∂        B                    ∂        t              ⁢    A  
The voltage generated by induction is proportional to the number of turns, N, in an electrical winding and the rate of flux change through those windings
                    ∂        B                    ∂        t              ⁢    A    ,where ∂B is the flux density change during the vibration and A is the area of the coil through which the flux change perpendicular to the coil plane is measured by the N turns.
In order to increase output voltage at a given frequency, either the product NA must be increased or the flux change ∂B must be increased. Consequently, the power produced by inductive energy harvesters is presently limited by coil size (NA), the magnitude of the vibration amplitudes and frequencies (to increase
      ∂    B        ∂    t  and the need for heavy, powerful permanent magnets to produce a large flux density change ∂B. Typical reported output voltages are low unless the device is large. For example, with a flux ∂B=0.5 tesla coupled to a 30 Hz vibration so that
            ∂      B              ∂      t        =            2      ⁢      π      ⁢                          ⁢      f      ⁢                          ⁢      Δ      ⁢                          ⁢      B        ≈    100  Tesla/second, a device with a one cm2 area sensed by a 1000 turn coil generates an induced voltage of approximately ten volts into an infinite load impedance. However, in a practical system, as the load impedance decreases, current flows and, in accordance with Lenz's law, generates a back EMF that opposes the motion of the magnet and opposes the induced voltage thereby reducing the power output. Consequently, typical systems described in the literature report an average power output of approximately only 0.3 microwatts in a small device (for example, see “Development of an Electromagnetic Microgenerator”, C. Shearwood and R. B. Yates, Electronics Letters, v. 13, p. 1883 (1997)). The maximum power output of small inductive energy harvesters has been estimated to be 400 microwatts (“Self-Powered Signal Processing Using Vibration-Based Power Generation”, R. Amirtharaja and A. Chandarakasan, IEEE Journal of Solid State Circuits, v. 33, n. 5, pp. 687-695 (1998). The size of these devices indicates that the power density that can be achieved by inductive harvesters is in the range of 0.005 to 0.5 milliwatts/cm3.
As a result, attempts have been made to vary the coil and magnet configuration to increase the power output. An example of a prior art device is disclosed in U.S. Published Patent Application No. 20020172060, which describes an inductive vibration energy harvester that also damps vibrations (as any energy harvester will, in proportion to the amount of energy it scavenges from the vibration source). The disclosed device consists of a dipole magnet and an induction coil that encircles the magnet close to the magnet midpoint. In the absence of vibration, the magnet and induction coil are held in relative position by a pair of coil springs. The two components move relative to each other under the action of the external vibration so that the induction coil generates electrical power as described above. In a second embodiment in the cited prior art patent, two orthogonal dipole magnets move independently along their respective axes, relative to two induction coils. In both embodiments, each coil contains a soft iron flux concentrator sleeve through which the magnet passes. However, the disclosed design does not result in a large flux change at the coil location in response to vibrations because the ends of the magnet, where the greatest flux change occurs, are positioned far from the coil. Accordingly, in experimental models constructed with this design, the flux change through the coil as a result of vibration is very small and power harvested from 60 Hz vibrations at a strength of about 0.5 g is limited to less than one milliwatt for harvesters measuring approximately 10 cm×10 cm×10 cm.
Accordingly, there is a need for an energy harvester with an increased efficiency and output.